Conventionally, an optical scanning mirror system is used in optical equipment such as a bar code reader or a projector so as to scan a light beam incoming on a mirror by swinging a mirror unit to which the mirror is provided. A compact type one having a semiconductor structure and formed with using micro-machining is known as an optical scanning mirror, for example. Such a semiconductor structure has a moving unit to which a mirror is formed and a fixed frame for supporting the moving unit, when it is used as the optical scanning mirror. The moving unit and the fixed frame are coupled with each other by hinges. A pair of comb electrodes interdigitated each other is formed between the moving unit and the fixed frame, for example. The comb electrodes are formed so that each electrode is interdigitated at an interval from 2 μm to 5 μm, and generate electrostatic force when a voltage is applied between the electrodes. The moving unit rotates relative to the fixed frame while twisting the hinges by driving force generated by the comb electrodes, so that it swings around the hinges as a rotation shaft.
By the way, as shown in a document (IEEE Journal of selected topics in Quantum Electronics, Vol. 6, No. 5, September/October 2000 p 715), there is an optical scanning mirror having a semiconductor structure that the moving unit has a mirror unit to which a mirror is mounted and a movable frame which supports the mirror unit through hinges, and a pair of comb electrodes is further formed between the movable frame and the mirror unit. FIG. 25 and FIG. 26 show an example of such a biaxial optical scanning mirror. An optical scanning mirror 81 is configured by an SOI (Silicon on Insulator) substrate 800 which is formed by joining a first silicon layer 800a and a second silicon layer 800b disposed below thereof through an insulation film 820. A mirror unit 82 and a movable frame 83 are formed on the first silicon layer 800a, and a fixed frame 84 is configured by the first silicon layer 800a, the insulation film 820 and the second silicon layer 800b. The movable frame 83 is pivoted on the fixed frame 84 through first hinges 85. The mirror unit 82 is pivoted on the movable frame 83 through second hinges 86 which are formed in a direction perpendicular to the first hinges 85. Comb electrodes 87, 88 are respectively provided between the movable frame 83 and the fixed frame 84 and between the mirror unit 82 and the movable frame 83. A mirror 82a is formed on an upper face of the mirror unit 82. Terminal regions 810a, 810b, 810c, to which voltages to drive the comb electrodes 87, 88, are formed on an upper face of the fixed frame 84. The upper face of the first silicon layer 800a except the terminal areas 810a, 810b, and 810c is covered by the insulation film 820. When voltages are applied to the terminal areas 810a, 810b, 810c, the comb electrodes 87, 88 generate driving forces, and the driving forces act on the mirror unit 82 and the movable frame 83 so that the mirror unit 82 and the movable frame 83 respectively swing while twisting the second hinges 86 and the first hinges 85.
In the semiconductor structure of the biaxial optical scanning mirror 81, it is necessary to provide to two regions which are electrically insulated each other in the movable frame 83 so as to enable to apply the voltages between the electrodes in the mirror unit 82 side and the electrodes in the movable frame 83 side of the comb electrodes 88 provided between the mirror unit 82 and the movable frame 83. In FIG. 25, the regions of the first silicon layer 800a which are electrically insulated are respectively patterned with different designs. In the conventional semiconductor structure of the optical scanning mirror 81, the movable frame 83 is insulated in two regions as illustrated in the figure, one of which is the region having the same potential as that of the electrodes in the movable frame 83 side, and the other is the region having the same potential as that of the electrodes in the mirror unit 82 side by conducting the mirror unit 82 through the second hinges 86, by providing isolation trench 89 on the moving mirror 83. Such isolation trench 89 are provided by forming insulation films 820c on side walls of each trench formed on the first silicon layer 800a and by filling polysilicon 89a into the trench, so as to maintain mechanical strength of an integration of the movable frame 83 by coupling the two regions in electrically insulated state. Thereby, it enables to swing the movable frame 83 in a unitized manner and to maintain the electric insulation of the two regions of the movable frame 83.
An example of manufacturing processes of the isolation trench 89 is described with reference to FIG. 27A to FIG. 27C. First, as shown in FIG. 27A, resists 832 are patterned on the upper face of the insulation film 820 on the first silicon layer 800a of the SOI substrate 800, and the first silicon layer 800a is etched so as to form the trenches 801a on the first silicon layer 800a. Subsequently, as shown in FIG. 27B, after removing the resists 832, the insulation films 820c are formed by oxidizing the side walls of the trenches 801a with using an electric furnace, and polysilicon is deposited to fill the trenches 801a by the polysilicon 89a. Subsequently, as shown in FIG. 27C, the polysilicon deposited on the surface of the first silicon layer 800a is removed by polishing, the isolation trench 89 are formed in the first silicon layer 800a. After that, by removing the second silicon layer 800b and the insulation film 820 just below the movable frame 83 and the mirror unit 82, the movable frame 83 and the mirror unit 82 are formed movably.
However, the manufacturing processes of the semiconductor structure become complicated when the isolation trench 89, which are formed by filling the polysilicon 89a into the trench 801a, is provided to form the movable frame 83. In addition, since it is difficult to maintain favorite electric insulation and to maintain mechanical strength due to providing the isolation trench 89, simultaneously, there is a problem that yield ratio of the products may be deteriorated. In other words, in manufacturing of the semiconductor structure of the optical scanning mirror 81, complicated processes such as a trench formation process, a side wall oxidation process, a polysilicon filling process, a polysilicon polishing process must be performed, as mentioned above. Furthermore, in the polysilicon filling process, it is difficult to fill the polysilicon 89a thickly into the trenches 801a, so that air gaps may occur in the filled polysilicon 89a, and thus, the mechanical strength of the movable frame 83 may be weaken. Still furthermore, since the two regions of the movable frame 83 are electrically insulated each other by the insulation films 820c, if the insulation films 820c are not formed preferably in the manufacturing processes, the electric insulation between the two regions may be deteriorated, and thus, malfunction may occur in the optical scanning mirror 81.
Furthermore, in case of using the above mentioned biaxial optical scanning mirror for raster scanning purpose, it is required to increased a number of scanning lines by making resonance frequency of the mirror unit 82 higher than resonance frequency of the movable frame so as to scan a precise image widely. However, since the mirror unit 82 and the movable frame 83 have substantially the same thickness in the conventional structure, it is necessary to upsize the movable frame in order to increase a ratio of the resonance frequencies of the mirror unit 82 and the movable frame 83, and thus, dimensions of the device of the optical scanning mirror 81 will be upsized and manufacturing cost thereof increases.
Still furthermore, the first hinges 85 of the movable frame 83 may be formed narrower, while the mass of the movable frame 83 is increased to make the resonance frequency thereof smaller than that of the mirror unit 82. Since the movable frame 83 is formed on the first silicon layer 800a which is made to thin a silicon substrate having a thickness of several hundreds μm to several tens μm, if the displacement of the movable frame 83 may be enlarged than that in the normal swing due to adding of large vibrations in handling the optical scanning mirror 81, a stress larger than breaking strength acts on the first hinges 85, and thus, the first hinges 85 may be damaged, and consequently, the optical scanning mirror 81 may be inoperative.